LTC1530CS8 datasheet(10/24 Pages) LINER

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LTC1530

Figure 5b is derived based on the condition that

ILMAX = ILOAD + IRIPPLE/2. Therefore, it only provides the

minimum RIMAX value. It must be understood that during

the initial power-up phase (VOUT = 0V), the initial start-up

ILMAX can be much higher than the steady state condition

ILMAX. Therefore, RIMAX must be selected with the start-up

ILMAX in mind. In general, high output capacitance com-

bined with a low value inductor increases the start-up

ILMAX. Figures 6a and 6b plot the start-up ILMAX vs output

capacitance and inductance for unloaded and loaded con-

ditions with the current limit circuit disabled. Figures 6a

and 6b are provided as examples. Actual ILMAX under

start-up conditions must be measured for any application

circuit so that RIMAX can be properly chosen.

In order for the current limit circuit to operate properly and

to obtain a reasonably accurate current limit threshold, the

IMAX and IFB pins must be Kelvin sensed at Q1’s drain and

source pins. A 0.1

µF decoupling capacitor can also be

connected across RIMAX to filter switching noise. In addi-

tion, LTC recommends that the voltage drop across the

RIMAX resistor be set to ≥100mV. Otherwise, noise spikes

or ringing at Q1’s source can cause the actual current limit

to be greater than the desired current limit set point.

MOSFET Gate Drive

The PVCC supply must be greater than the input supply

voltage, VIN, by at least one power MOSFET VGS(ON) for

efficient operation. This higher voltage can be supplied

with a separate supply, or it can be generated using a

simple charge pump as shown in Figure 7. The 86%

maximum duty cycle ensures sufficient off-time to refresh

the charge pump during each cycle.

As PVCC is powered up from 0V, the LTC1530 undervoltage

lockout circuit prevents G1 and G2 from pulling high until

PVCC reaches about 3.5V. To prevent Q1’s high RDS(ON)

from triggering the current limit comparator while PVCC is

slewing, the current limit circuit is disabled until PVCC is

≥8V. In addition, on start-up or recovery from thermal

shutdown, the driver logic is designed to hold G2 low until

G1 first goes high.

ILMAX (A)

5500

4500

3500

2500

1500

500

16 18

1530 F05b

810

RIMAX ≥ 500Ω

ILMAX = ILOAD + IRIPPLE/2

Q1 RDS(ON) = 0.05Ω

0.04

Ω

0.03

Ω

0.02

Ω

0.01

Ω

Figure 5b. Minimum Required RIMAX vs ILMAX

OUTPUT CAPACITANCE (mF)

1530 F06a

TA = 25°C

VIN = 5V

ILOAD = 0A

L = 1.2

µH

L = 4.7

µH

L = 2.4

µH

Figure 6a. Start-Up ILMAX vs Output Capacitance

OUTPUT CAPACITANCE (mF)

1530 F06b

TA = 25°C

VIN = 5V

ILOAD = 10A

L = 1.2

µH

L = 4.7

µH

L = 2.4

µH

Figure 6b. Start-Up ILMAX vs Output Capacitance

APPLICATIO S I FOR ATIO

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